1. Field of the Invention
The invention relates to electro-optical switches, and in particular to switches which exhibit a step-like response to switching voltage.
2. Description of the Prior Art
The present invention relates to electro-optical switches for switching a beam of optical radiation between two waveguides on a substrate. Optical waveguides are typically fabricated from a pyroelectric material such as lithium niobate or lithium tantalate or from semiconductor materials such as gallium arsenide or indium phosphide. Wavepaths or waveguides are generally fabricated in the substance by depositing a dopant such as titanium on the surface of the substrate in the pattern desired for the wavepaths. The substrate is then heated to diffuse the dopant into the substrate. This procedure forms a wavepath or waveguide, i.e., a section in the pyroelectric material that guides light, usually about 3 to 10 .mu.m wide. To permit logic operations, signal processing, or switching between wavepaths in the pyroelectric material, an electric field is imposed across the region of the crystal where a change in beam direction is desired. Such a field is generated by electrodes deposited on the substrate for this purpose. The field produces local changes in the optical polarizability of the crystal, thus locally changing the refractive index and, in turn, altering the path of light through the crystal.
In one form of integrated optics switch and modulator, known as a directional coupler switch, a pair of waveguides of refractive index greater than the surrounding substrate are provided in close parallel relationship to one another with electrodes provided thereon. Unfortunately, in many substances such as lithium niobate (LiNbO.sub.3) and lithium tantalate (LiTaO.sub.3) the modulators and switches so formed have proven to be polarization sensitive. For example, while light having a direction of polarization perpendicular to the substrate plane can be substantially modulated or switched, light with a polarization parallel to the substrate plane is simultaneously disadvantageously unaffected or relatively unaffected.
Switches in the form of directional couplers, preferably manufactured on a GaAs base or a LiNbO.sub.3 base, and whereby the control arrangement essentially is formed by a single controllable DC voltage supply are disclosed in:
1. "Optical Waveguide Connecting Networks", H. F. Taylor, Electronics Letters, Vol. 10, No. 4, Feb. 21, 1974, pp. 41-43;
2. "Electro-Optic Y-Junction Modulator/Switch", Electronics Letters, Vol. 12, No. 18, Sept. 2, 1977, pp. 459-460;
3. "Experimental 4.times.4 Optical Switching Network", Schmidt et al, Electronics Letters, Vol. 12, No. 22, Oct. 28, 1976, pp. 575-577;
4. "Metal-Diffused Optical Waveguides in LiNbO.sub.3 ", Schmidt et al, Applied Physics Letters, Vol. 25, No. 8, Oct. 15, 1974, pp. 458-460;
5. "A New Light Modulator Using Perturbation of Synchronism Between Two Coupled Guides", Tada et al, Applied Physics Letters, Vol. 25, No. 10, Nov. 15, 1974, pp. 561-562;
6. "GaAs Electro-Optic Directional-Coupler Switch", Campbell et al, Applied Physics Letters, Vol. 27, No. 4, Aug. 15, 1975, pp. 202-205;
7. "Electrically Switched Optical Directional Coupler: Cobra", Papuchon et al., Applied Physics Letters, Vol. 27, No. 5, Sept. 1, 1975, pp. 289-291;
8. "Electro-Optically Switched Coupler with Stepped DELTA Beta Reversal Using T.sub.i -diffused LiNbO.sub.3 Waveguides", Schmidt et al, Applied Physics Letters, Vol. 28, No. 9, May 1, 1976, pp. 503-506;
9. "Electrically Active Optical Bifurcation: BOA", Papuchon et al, Applied Physics Letters, Vol. 31, No. 4, Aug. 15, 1977, pp. 266-267.
Electro-optic switches are interferometric in nature, i.e., they require a precise phase shift to achieve a switched state with low crossstalk. The directional coupler switch, for example, requires a phase shift of 3" between its two waveguides to switch. Because of small fabrication errors, this phase shift requires slightly different voltages for each switching element in a switching array. It is also very difficult to obtain switching in the two orthogonal polarizations simultaneously.
Other electro-optical switches are described in:
1. "Guided-Wave Devices for Optical Communication", R. C. Alferness, IEEE Journal of Quantum Electronics, Vol. QE-17, No. 6, June 1981, pp. 946-958;
2. "Polarization-Independent Optical Directional Coupler Switch Using Weighted Coupling", R. C. Alferness, Appl. Phys. Lett. 35 (10), 15 Nov. 1979 pp. 748-750;
3. "Optical-Waveguide Hybrid Coupler," M. Izutsu, A. Enokihara, T. Sueta, Optics Letters, November 1982 Vol. 7, No. 11 pp. 549-551;
4. "Electrically Active Optical Bifurcation: BOA" M. Papuchon and A. M. Roy, Appl. Physics Letters, Vol. 31, No. 4 15 Aug. 1977, pp. 266-267;
5. "Digital-Controlled Electro-optic 1.times.2 Switch for Time-Division Multiplexing and Data Encoding", K. T. Koai and P. L. Liu, Applied Optics, Vol. 25, No. 22, 15 Nov., 1986, pp. 3968-3970; "4.times.4 Ti: LiNbO.sub.3 Integrated Optical Crossbar Switch Array", L. McCaughan and G. A. Bogert, Appl. Phys. Lett., 47 (4), 15 Aug. 1985, pp. 348-350; The most useful switch by far has been the directional coupler switch. It has the following drawbacks:
1. It requires a precise voltage to define the switch states. This voltage may change slightly in time, and vary from device to device due to fabrication variations.
2. It usually operates only on one polarization state. It is very difficult (although demonstrated) to build a polarization insensitive directional coupler.
3. It is wavelength sensitive; it will operate satisfactorily only on a narrow wavelength range.
4. It is sensitive to fabrication tolerance. Some of this sensitivity may be removed by using a "delta beta reversal" switch (Reference 8) at the price of having to control a voltage source for precise switching.
Other switches suffer from similar problems. The proposal to synthesize a switch which will have a digital response has also been made. This synthetic switch is very complicated and still has many drawbacks. Prior to the present invention, there has not been a switch which is simultaneously:
1. Insensitive to switching voltage variations. (This makes this switch particularly attractive for use in switch arrays where a single voltage source can be used.)
2. Insensitive to fabrication variations.
3. Provides simultaneous switching of two orthogonal polarization components.
4. Permits operation over a broad wavelength range.